1. Field of the Invention
The present invention relates to the improved free-radical polymerization of chloroprene or copolymerization of chloroprene with appropriate comonomers in aqueous emulsion, in the presence of an emulsifying system which does not contain carboxylic acid soaps.
2. Description of the Prior Art
The chloroprene polymers are formed in a conventional manner by emulsion polymerization, in the presence of emulsifiers. When the polymerization is carried out in an alkaline medium, the emulsifiers usually comprise alkali metal salts of colophony.
However, for certain applications, such as formulations for adhesives comprising a solvent, the presence of colophony in too high an amount is liable to cause a separation of the phases in the presence of metal oxides and is therefore not desirable. When polymerization in an acid medium is necessary, for example in order to copolymerize chloroprene with olefinically unsaturated carboxylic acids, the use of colophony is not possible because of the pKa of the corresponding resinic acids. Other ionic surfactants must then be used, such as those containing sulfate or sulfonate groups.
FR-A-2,333,818 describes a process for the production of concentrated polychloroprene latexes using, per 100 parts of monomers, from 3 to 6 parts by weight of ionic emulsifiers, if necessary conjointly with 0.5 to 6 parts by weight of nonionic emulsifiers. These relatively high proportions of ionic emulsifier of the sulfonate or sulfate type present several disadvantages. During the polymerization, the exothermic reaction is difficult to control, with the possibility that the heat generated will exceed the heat transfer capacity of the polymerization apparatus. Control of the reaction is the more difficult the higher the concentration of monomers in the aqueous phase; however, for numerous applications it is desirable to use a latex having a high solids content, preferably of at least 55%.
It is also known that the conjoint addition of a nonionic agent permits costabilization of the emulsion and lowering of the gel point. According to the aforesaid published French application, the amount of ionic emulsifier may, however, not be decreased to a proportion of below 3% of the polymerizable monomers. Moreover, the use of large amounts of nonionic compounds has adverse consequences as regards the reaction kinetics and for the control of the particle size distribution.
Another disadvantage of the use of high proportions of ionic emulsifier of the sulfate or sulfonate type is the fact that it is difficult, or even impossible, to destabilize the resulting latex by means of low temperature in order to isolate the polymer.
On the other hand, the use of latexes in this state may present another problem in certain cases because of a lack of colloidal stability in the presence of polyvalent metal ions. In certain fields of application, such as, for example, reaggregation, impregnation or aqueous adhesive formulations, it is desirable that the latex does not coagulate prematurely in the presence of ions such as Ca.sup.++, Zn.sup.++, Al.sup.+++ or others which may be present in the respective compositions.
FR-A-2,231,725 and CA-A-1,031,489 describe a process for preparing polychloroprene latexes which do not coagulate during the addition of electrolytes or other materials present in the latex-based compositions, for example zinc oxide. According to such process, the chloroprene is polymerized in an emulsion devoid of carboxylic soaps but in the presence of a polyvinyl alcohol. The latex obtained lacks stability on storage as a consequence of hydrolysis of the colloid.